Consider two relations which are both non$-$empty.

$R1,$ which has columns $\{x,y\},$ and primary key $\{x\}$

$R2,$ which has columns $\{y\},$ and primary key $\{y\}$

Which of the following must be true? Select all that apply.

a$){}_{y}R1$ will never have duplicate rows

b$){}_{x}R1$ will never have duplicate rows

c$){}_{x}R1$ will have the same number of rows as ${}_{y}R1$

d$){}_{y}R1R2$ will be non$-$empty

e$){}_{y}R1R2$ will be non$-$empty

f$)R1-R2$ is a valid expression